Aluminum and aluminum alloys have excellent properties including light weight, high corrosion resistance and high thermal conductivity. Therefore, they have been widely applied to products which are required to have the above properties, such as aircraft, automobiles and other mechanical components.
However, the aluminum and the aluminum alloys have poor properties such as low strength, especially at the temperature of 200.degree. C. or more, high coefficient of thermal expansion and low modulus of rigidity. These defects limit the applications of the aluminum and the aluminum alloys.
For improving the defects of the aluminum and the aluminum alloys, aluminum matrix composites comprising the ceramic particles dispersed in the matrices of aluminum or aluminum alloys are developed.
As methods for preparing the aluminum matrix composite comprising ceramic particles, three methods are known. The first method comprises impregnating a molten aluminum or aluminum alloy into a preform formed from the ceramic particles (please see Japanese Patent kokai No. 89/306506). A part of the composite prepared according to the first method are commercialized. In practice, the reason that the ceramic content should be selected to be relatively high (generally 20% by volume or more) for forming the preform limits the application of the first method. The second method comprises mixing the aluminum or aluminum alloy powder with the ceramic particles under a dry condition (please see Japanese Patent kokai No. 91/122201). Although the ceramic content can be suitably selected, the second method is not practically applied, because the uniform mixture of the aluminum or aluminum alloy powder with the ceramic particles is technically very difficult. The third method comprises dispersing the ceramic particles in the molten aluminum or aluminum alloy (please see Japanese Patent kohyo No. 89/501489). In the third method, the ceramic content can be suitably selected and the dispersion of the ceramic particles in the molten aluminum or aluminum alloy is relatively uniform as compared with the mixture of the second method. However, the third method is not practically applied, because as shown in the following comparative example, alloying elements and the ceramic particles may segregate near grain boundaries and/or they may not uniformly dispersed due to a slower solidification rate, thereby a product resulting from this composite has poor mechanical properties.